Electrophotographic imaging processes and techniques have been extensively described in patents and other literature. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, well-known in the art, can then be employed to produce a visible record of the electrostatic image.
A group of important electrophotographic elements used in these electrophotographic imaging processes comprise a conductive support in electrical contact with a charge generation layer (CGL) and a charge transport layer (CTL). Electrophotographic elements having at least one of the layers designed primarily for the photogeneration of charge carriers (holes and electrons), referred to as CGL, and at least one other layer designed primarily for the transport of the generated charge carriers, referred to as CTL, are sometimes referred to as multilayer or multiactive electrophotographic elements. Representative patent publications disclosing methods and materials for making and using such elements include U.S. Pat. Nos. 4,495,263; 4,701,396; 4,666,802; 4,427,139; 3,615,414; 4,175,960 and 4,082,551.
Photoconductive elements of this type have found widespread use in xerography. One continuing problem is that the lifetime of these elements is less than desired. The photoconductive elements are cycled in the machine and go through all of the steps in the electrophotographic imaging process many times. The corona charging and cleaning steps in the process are particularly damaging to the photoconductive element. It is believed that corona charging causes the formation of chemical species on the photoconductive element surface which causes a problem called "image spread". In Order to overcome this problem, the cleaning step is designed to remove these species. While removing these species, the cleaning step also removes a surface layer of the photoconductive element. Thus, a small amount of photoconductive element is removed with each cycle. Eventually, the photoconductor will be worn to the point of needing replacement.
Further, organic photoconductive elements are scratch prone. Extreme care must be taken when installing replacement elements since any scratches show up as defects on copies.
It has been proposed to put protective overcoats on photoconductive elements of this general type. For example, U.S. Pat. No. 4,965,156 to Hotomi et al. discloses the use of two protective layers on an organic photoconductive element. The first layer is an amorphous carbon layer which includes more than 5 atomic percent fluorine. The second, outermost layer is a similar material except that the fluorine content must be lower than 5 atomic percent. Hotomi et al. teach that if the fluorine content is above 5 atomic percent in the outermost layer, it causes image fogging. The photoconductor of interest to Hotomi et al. includes a hydrazone compound in the charge transport layer.
Arylamines used as charge transport materials in photoconductive elements are described in U.S. Pat. Nos. 4,127,412 and 5,190,840 and are used commercially.
One potentially useful protective coating for electrophotographic elements would be a diamond-like carbon coating; however, such coatings, while providing for physical protection from scratches, also have high surface energies. High surface energies make the removal of toner from the photoconductive element difficult and decrease the efficiency of processes involved in the removal of toner from the photoconductive member, such as cleaning, resulting in image defects and in the transfer of unwanted toner to subsequent copies, reducing copy quality. It is not apparent what modification can be made to such coatings to reduce or eliminate this problem.
Accordingly, there is a continuing need for photoconductive elements of this type which have longer process lifetimes and low surface energy.